Up to now, many methods have been proposed for preparing a flame-retarding fiber and one of them is a post-processing method where a flame retardant such as phosphorus compound or halogen compound is adhered and fixed on the surface of the fiber but, in that method, it is difficult to adhere a lot of the flame retardant as such so that it is difficult to prepare a fiber having a high flame-retarding property and, in addition, there are various problems such as durability, changes in texture and toxicity of the flame retardant itself and also upon burning.
A representative example of other methods is a method where fiber is formed using a polymer in which a halide monomer such as vinyl halide and vinylidene halide is copolymerized but, in that method, it is necessary to copolymerize a halide monomer in large amount for preparing a highly flame-retarding fiber and, as a result, there is also an essential disadvantage such as generation of noxious gases upon burning.
Against the problems as such, flame-retarding fibers in which carboxyl group obtained by a hydrolysis reaction of a cross-linked acrylate fiber is cross-linked by a polyvalent metal ion such as zinc, copper, calcium and iron have been proposed in Japanese Patent Laid-Open Nos. 01/314,780, 02/084,528 and 02/084,532. However, a limiting oxygen index (hereinafter, it will be referred to as LOI) showing the degree of flame-retarding property is as high as 37 in the fiber where vinylidene chloride which is a halide monomer is used but, when no halide monomer is used, it is 34 at the highest.
In Japanese Patent Laid-Open No. 04/185,764, there is a proposal for a cross-linked acrylate fiber where an increase in nitrogen content by hydrazine cross-linking is more than a predetermined value in which ionic cross-linking is conducted by copper ion. In that case, a product of a high flame-retarding property where the LOI is up to 35 at the highest is able to be prepared. However, since it uses copper, copper ion which is heavy metal ion causes a problem in its discarding or in discarding after burning.
In the Japanese Patent Laid-Open Nos. 08/325,938 and 09/059,872, there is a description for a moisture-absorptive fiber having a flame-retarding property where carboxyl group is introduced by hydrolysis into an acrylate fiber into which cross-link has been introduced by hydrazine and said carboxyl group is made into a metal salt type selected from the group consisting of calcium, magnesium, aluminum, copper, zinc and iron salt type. However, in the fiber of a calcium salt type disclosed in Examples of those documents, LOI is 30 at the highest and no high flame-retarding property is bestowed. Although moisture-absorptive property is said to be one of the characteristics therein, the moisture-absorptive rate at 20° C. and 65% relative humidity is about 30% at the highest and very high property is not achieved.
In Japanese Patent Laid-Open No. 10/237,743, there are examples for pile cloth as a structure comprising an acrylate fiber where hydrogen and at least one type of metal selected from calcium, magnesium and aluminum are bonded to carboxyl group. However, LOI of the flame-retarding acrylate fiber (trade name: “eks” (trade mark) manufactured by Toyobo) disclosed in Examples of said document is 31 at the highest and the fiber is not highly flame-retarding.